There is a projected global shortage for benzene which is needed in the manufacture of key petrochemicals such as styrene, phenol, nylon and polyurethanes, among others. Generally, benzene and other aromatic hydrocarbons are obtained by separating a feedstock fraction which is rich in aromatic compounds, such as reformate produced through a catalytic reforming process and pyrolysis gasolines produced through a naphtha cracking process, from non-aromatic hydrocarbons using a solvent extraction process.
To meet this projected supply shortage, numerous catalysts and processes for on-purpose production of aromatics (including benzene) from alkanes containing six or less carbon atoms per molecule have been investigated. These catalysts are usually bifunctional, containing a zeolite or molecular sieve material to provide acidity and one or more metals such as Pt, Ga, Zn, Mo, etc. to provide dehydrogenation activity. For example, U.S. Pat. No. 4,350,835 describes a process for converting ethane-containing gaseous feeds to aromatics using a crystalline zeolite catalyst of the ZSM-5-type family containing a minor amount of Ga. As another example, U.S. Pat. No. 7,186,871 describes aromatization of C1-C4 alkanes using a catalyst containing Pt and ZSM-5.
Most lower alkane dehydroaromatization processes carry out the reaction in one step. For example, EP0147111 describes an aromatization process wherein a C3-C4 feed is mixed with ethane and all are reacted together in a single reactor. A minority of these processes involves two separate steps or stages. For example, U.S. Pat. No. 3,827,968 describes a process which involves oligomerization followed by aromatization. U.S. Pat. No. 4,554,393 and U.S. Pat. No. 4,861,932 describe two-step processes for propane involving dehydrogenation followed by aromatization. None of these examples mention a two-stage process in which lower alkane aromatization takes place in both stages.
The ease of conversion of individual alkanes to aromatics increases with increasing carbon number. When a mixed feed consisting of ethane and higher hydrocarbons is converted into benzene plus higher aromatics in a single stage, the selection of reaction severity is dictated by the desired overall hydrocarbon conversion target. If a significant level of ethane conversion is desired or needed, this may lead to operating such a one-stage process at higher temperature severity. The negative consequence of this higher severity is such that higher hydrocarbons such as propane can undergo non-selective side reactions which result in excessive hydrogenolysis into lower-valued methane. The net result is that the overall yield to benzene and other aromatics is reduced significantly.
It would be advantageous to provide a light alkane dehydroaromatization process wherein (a) the conversion of each component of a mixed alkane feed can be optimized, (b) the ultimate yield of benzene is greater than that of any other single aromatic product, and (c) the generation of undesired methane by-product is minimized.